B7-1 (CD80) is a T cell costimulatory molecule that is found on the surface of antigen presenting cells (APCs). Originally described as a cell adhesion molecule, it is now known that B7-1 sends important costimulatory signals through its two T cell surface receptors, CD28 and CTLA4 (CD152). B7-1 interacts with CD28 to signal cytokine production, cell proliferation, and the generation of effector and memory T cells. If the signal through CD28 is blocked T cell, anergy or immune deviation can occur, resulting in severely depressed or altered immune response. For example, when B7-1 interaction with CD28 (and CTLA40) is blocked with a soluble CTLA4Ig, allograft tolerance and resistance to autoimmune diseases have been observed.
B7-1 also interacts with the T cell CTLA4 receptor. Its signaling is complex, but one component provides a negative feedback signal, causing the T cell to attenuate the CD28 signal. In the absence of this signal for a long period of time, rampant T cell proliferation and effector cell activation continues. However, shorter term intervention can be beneficial by leading to a more vigorous immune response. For example, when the interaction of B7-1 is blocked with antibodies to CTLA4, increased rejection of tumors has been found. When this feedback regulation malfunctions, autoimmune diseases and lymphoproliferation can result. For example, when the CTLA4 and B7-1 interaction is blocked with a soluble CTLA4Ig, allograft tolerance and resistance to autoimmune diseases have been observed.
In addition to B7-1, other molecules are known to send costimulatory signals to T cells. For example, B7-2 (CD86), which is expressed on different cells and at different stages of APC activation from that of B7-1, also delivers its costimulatory signal to T cells through CD28 and CTLA4. The B7-2 signal can lead to immune responses that are identical to, or different from the immune responses resulting from B7-1 signaling. The nature of the B7-2 signaling depends upon the cellular context and the timing of the costimulation.
Some evidence suggests that additional molecules bind CTLA4. Evidence also exists that other molecules are involved in sending important CD28-independent costimulatory signals to T cells.
Even though they bind to the same cellular receptors, B7-1 and B7-2 are only weakly related at the amino acid level. Both, however, are members of the extended immunoglobulin domain-containing superfamily and much of their shared sequence homology is due to the particular residues shared by their common Ig domains, which are characteristic of the Ig-domain subfamily.
Clearly, costimulatory signaling through T cell surface receptors plays an important role in maintaining balance in the immune system. Systems with a predominance of activatory signals, such as the costimulatory signaling between CD28 and B7-1, can lead to autoimmunity and inflammation. Immune systems with a predominance of inhibitory signals, such as the costimulatory signaling between CTLA4 and ??? are less able to challenge infected cells or cancer cells. Isolating new molecules involved in costimulatory signaling is highly desirable for studying the biological signal(s) transduced via the receptor. Additionally, identifying such molecules provides a means of regulating and treating diseased states associated with autoimmunity, inflammation and infection. For example, engaging a molecule that stimulates inhibitory or negative signaling with an agonistic antibody or signaling partner can be used to downregulate a cell function in disease states in which the immune system is overactive and excessive inflammation or immunopathology is present. On the other hand, using an antagonistic antibody specific for a molecule that stimulates negative signaling, or using a soluble form of the molecule to block signaling, can activate the specific immune function in disease states associated with suppressed immune function. Conversely, engaging a molecule that stimulates positive signaling with an agonistic antibody can be used to upregulate the effect of that molecule's signaling.
In view of the evidence that undefined T cell costimulatory molecules exist and further in view of the continuing search for new therapeutics for treating infection, autoimmune diseases, and inflammation, it would be desirable to identify additional T-cell costimulatory molecules. In particular there is a need for additional molecules that alter T cell costimulation during an in vivo immune response.